For rotating and reciprocating equipment, non-intrusive monitoring systems are commonly used in applications where real time monitoring of the rotating and reciprocating equipment on process plants is impeded by long distances or difficulty of access. Efficient operation and maintenance of rotating and reciprocating equipment is essential to maximize production and minimize downtime. Non-intrusive monitoring systems are used to detect or predict equipment defects before catastrophic failure of the equipment occurs, which would result in loss of production capacity and possible injury of personnel.
It is desirable to detect and locate equipment defects while the equipment is in-situ so as not to interfere with the production. Removing equipment from the production for routine inspection is undesirable, as production is lost during shutdown.
Acoustic emission transducers and apparatuses to monitor specific applications and determine failure of components related to rotating equipment and machinery have been developed in the past. U.S. Pat. No. 4,493,042 to Shima et al. presents the application of acoustic monitoring to detect and judge failures of roller bearings. Other inventions were made in developing specific signal processing algorithm to determine component failure based on acoustic emission data EP2031386 A1.
Generally the acoustic emission technology used hitherto for detecting failure of components related to rotating equipment and machinery use acoustic emission sensors that are placed on a component or component surrounding structure to detect sound-waves that are transmitted through the structure (structural acoustic emission sensors).
Power transmission couplings are components that transmit torque at a speed ratio of 1:1 between the shaft ends of a driving and driven machine. They are incorporated in the drive train to compensate small misalignments between the shaft ends due to mounting tolerances and operational displacements of the shafts and minimize the bearing loads associated with the misalignment. One of the most common industrial applications of couplings is their use in refineries to connect driver and pump or driver and compressor.
In a typical power transmission coupling as illustrated in cross section in FIG. 1, a hub 102 or adapter is provided on the end of a shaft on both driven and driver equipment and a transmission unit 104 connects the hubs 102 together to transmit drive and torque from the driver equipment to the driven equipment. A flexible assembly 106 is provided as an interface between each hub 102 and transmission unit 104 to absorb angular, radial and axial misalignment between the driven and driver equipment. An example of a flexible assembly 106 is the flexible membranes found in John Crane® T Series™ and M Series™ couplings, in which the flexible assembly 106 comprises a series of flexible elements 108 as illustrated in FIG. 2. The flexible elements 108 are stacked together on juxtaposed engagement, the flexible assembly 106 being secured alternately to a hub 102 and the transmission unit 104 by an even number of bolts 110, 112, which pass through holes 114 spaced angularly about the flexible elements 108.
During each shaft revolution the flexible assembly (106) and individual flexible elements (108) are exposed to torsional stresses due to the drive torque and bending stresses due to shaft misalignment.
When operating a coupling within the specified design limits, the flexible elements achieve a theoretical infinite service life of more than 106 load cycles. However, if conditions exceed the specified limit, operation beyond the misalignment limit and/or torque transmission beyond the design limit, the coupling will eventually fail due to fatigue stress cracks in the flexible elements 108 of the flexible assembly 106.
Such failure, in most cases developing over several days (weeks) from the onset of the first crack, could have costly consequences due to secondary damage to the machine or drive, production interruption and in some cases posing a severe health and safety risk.
Because each flexible assembly 106 comprises a series of individual flexible elements 108, it is difficult to detect failure of an individual flexible element 108 of flexible assembly 106. Each flexible element 108 during operation emits a different acoustic trace or signal.
Most importantly, initiation of the failure of the flexible assembly 106 starts with fretting, i.e. rubbing between individual flexible elements 108 followed by failure of a first flexible element 108 followed by failure of a second flexible element 108 and so forth. Therefore, the coupling is able to function for some time before catastrophic failure of the flexible assembly 106.
Detecting acoustic emissions emitted by a defect in the flexible element 108 of a coupling 100 using structural acoustic emission is unlikely to be successful and would not be possible with any of the existing detection technologies because a structural acoustic emission sensor cannot be placed close to the coupling's membrane unit but needs to be placed at some distance away on the machinery casing, where the sound consequently has to pass several component interfaces that eliminate the chance of detecting the signal within the noise of the surrounding machinery (bearing noise, process noise etc). For example with the coupling shown in FIG. 1, with structural acoustic emission sensors as used with detection apparatuses described in U.S. Pat. No. 4,493,042 to Shima et al., the sound would need to be transmitted from the flexible element 108 to the bolts 110, 112, from the bolts 110, 112 to the hub 102, from the hub to the machine shaft, from the shaft to a connecting bearing and from the bearing, which is a strong source of acoustic emission too, to the casing where the structural sensor is placed.